The problem of fresh water becomes more acute each year in many regions of the world as a result of population growth, climate changes and environmental pollution. In many regions of the world there is insufficient water to satisfy the needs of agriculture, industry, and even personal needs for drinking water. In other regions where water is available, it is not potable and has to undergo expensive processing, which can be environmentally polluting, to be useful.
The aquifers of the world on which agriculture depends in drier regions are being drained of their ground water faster than the reserves can be replaced by natural percolation of rainfall and runoff. For example, the Ogallala aquifer, a principal water source of the central United States, experienced a three-meter drop through one-fifth of its area during the 1980s alone. Now it is depleted by about a million hectares. Still worse deficits are building in other countries which can least afford it. Beijing's aquifer fell 37 meters in the past 30 years, while the ground water reserves of the Arabian peninsula are expected to be exhausted by 2050. On a global scale, humanity is reaching the limit of readily available water supplies, now using more than half that available in rivers and other runoff channels. By 2025, forty percent of the world's population will be living in countries with chronic water scarcity.
The main sources of fresh water are rivers, lakes and artesian wells. The costs of producing purified water from conventional purification plants from these sources is about twenty to fifty cents per cubic meter.
Alternative methods of water production include desalinization for production of fresh water from seawater using distillation, electrodialysis and reverse osmosis. Limitations of desalination include the need to locate the plant on the seaside, requiring a major transport system for application of the water inland for agricultural or municipal purposes. In addition, desalinization plants may cause air pollution from the burning of fossil fuels to drive the system and environmental pollution from the disposal of accumulated salt brine. The cost of fresh water from desalination plants is between about $1.20 and $1.80 per cubic meter.
Each year 577,000 km.sup.3 of water evaporates from the land and ocean surfaces. This major source of renewable fresh water, atmospheric water, is hardly being used. Half the total atmospheric water is contained in the lower 1.5 km of the atmosphere, and more than 99% in the troposphere. The mean absolute humidity near the earth's surface is 11 g/m.sup.3. In the tropics it is 25 g/m.sup.3 or even higher. For example in Djibouti there is almost no annual rainfall, but the humidity is 18-24 g/m.sup.3. The amount of water moving above each ten square meters in the Arabian and Sahara deserts has the volume of a lake having an area of about one square kilometer and a depth of 50 meters. In Israel 190 to 200 nights are characterized by 100% humidity.
In Feodosiya Russia, an ancient system of fountains, built c. 300 to 1800 AD, used water collected from detritus heaps in addition to wells and mountain runoff. These detritus heaps, two to five meters high and one to two thousand cubic meters or more in volume, were used to collect rainwater, and in drought times they condensed moisture from the atmosphere. The heaps were cone-shaped with a deep crater at the top and horseshoe-shaped hollows in the direction of drainage. Because of this shape, maximum heating of the heap corresponded to minimum heating area, so inside the heap the temperature was extremely different at different times of day, giving rise to convention flows inside the heap that resulted in moist air condensation.
Modern-day efforts to take advantage of atmospheric water have been underway in the Soviet Union since the mid-1930s. Systems with production capacities of up to 2000 m.sup.3 per day have been developed for large municipal or agricultural applications in the Crimean region.
German patent application No. 3313711 discloses a plant for producing fresh water from moist air using solar energy. The plant uses solar batteries, a refrigerating unit, a water header for collecting water, and an air duct in which an evaporator of the refrigerating unit and a fan are placed. This plant operates by using electrical power from solar batteries to power a refrigerating unit which feeds cold air into a condenser. Using a fan, moist air is blown through an air duct in which the condenser is placed. As a result of contact with the condenser surface, the air is cooled and the contained water vapor becomes saturated, is partially condensed on the condenser surface, and drains down into the water header.
Russian Federation Patent No. 2056479 discloses a plant for producing water from atmospheric air by accumulating cold at night. The plant comprises solar batteries, a refrigerating unit, a cold accumulator in the form of a heat-insulated tank filled with water connected through a hydraulic pump and a valve to the refrigerating unit, and a condenser placed in the air duct together with a drop catcher and a fan. A water header is located under the opening in the air duct. During the day, electrical power from solar batteries is delivered to the refrigerating unit which generates cold. The refrigerating unit is connected to a heat-insulated tank through a valve. Using a hydraulic pump, the water contained in the tank is pumped through the refrigerating unit and cooled, with the resulting accumulation of cold in the heat-insulated tank. Then the tank is disconnected from the refrigerating unit using a valve and connected to the condenser. As soon as the air moisture becomes close to 100%, the hydraulic pump and fan are switched on in order to pump cold water and moist air through the condenser. The water vapor in the air is condensed on the condenser surface, and the drops contained in it are caught by the drop catcher and drain down to a water header.
Drawbacks of these systems are that they require the use of fans, pumps and refrigerating units requiring electrical power.
It is an object of the present invention to provide a system for producing fresh water from atmospheric air not requiring the use of electrical or other artificial energy. Using the systems of this invention, fresh water can be produced at a cost of about ten to twenty cents per cubic meter.